The invention releates generally to acoustic impedance measuring devices and more specifically concerns measuring the specific acoustic impedance of the Earth's ground surface, or other surfaces, over a continuous band of frequencies in the low audio range.
The "acoustic impedance" of a surface is defined as the ratio of the sound pressure to the incident volume velocity of the air above the surface. This is also called the acoustic "surface impedance," or in the case of the Earth's ground surface, the acoustic "ground impedance". The "volume velocity" is defined as the acoustical particle velocity of the air times its cross-sectional area, in other words, the volume of air displaced past a reference plane per unit time. The "specific acoustic impedance" of the ground surface is the ground impedance multiplied by its test area.
Previous methods for performing the function of the invention can be organized into the following cateories: (1) impedance tube and waveguide methods in general; (2) free field methods, of which there are several variations based on the type of excitation (steady state harmonic, steady state wideband, or transient), the angle of incidence (normal or oblique), the number of microphones (usually one or two), and the parameter-evaluation technique (Fourier transform, transfer function, or curve fitting to multiparameter models); and (3) direct sound pressure-volume velocity measurement methods, in which the volume velocity is measured by means of a hot-wire anemometer or magnetic search coil.
The impedance tube suffers a major disadvantage in field applications: it requires an accurate measurement of the distance from the first interference minimum to an ill-defined test surface. Furthermore, because each frequency of operation requires several sound pressure measurements, the method is time-consuming and often introduces modifications of the test surface during the course of the measurement.
Free field methods suffer the disadvantage that free field sound propagation is subject to environmental distrubances such as wind, turbulence, thermal gradients, and background noise, and is sensitive to landscape features. Some of these methods utilize wave interference and are subject to the same disadvantages as the impedance tube. Some are mathematically intricate, relying on questionable assumptions, such as the planarity or sphericity of the wave front.
The major difficulty with direct sound pressure--volume velocity measurement methods lies in the measurement of volume velocity. The hot-wire anemometer is difficult to calibrate in the absence of mean flow and is not well suited for purely acoustical excitation. A magnetic search coil mounted on the back surface of an electrodynamically driven piston will cause measurement errors due to dynamic effects and due to mutual inductance with the drive coil, and will be relatively insensitive at low frequencies.
It is an object of this invention to provide a method and device for accurately measuring the acoustic impedance of a surface that do not have the disadvantages of the prior art methods and devices.
Another object of the invention is to provide a simple, compact and portable device for measuring the acoustic impedance of a ground surface.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.